1. Field of the Invention
The present invention relates to integrated circuits, and more particularly to integrated circuits having monolithic passive components.
2. Description of Related Art
There are numerous advantages to integrating not only the transistors but also the inductors and other passive components of a circuit onto a single monolithic substrate. For example, the manufacturing costs and the power consumption of the circuit can be substantially reduced by integrating the circuit onto a single chip. However, in some applications, it has not been practical to integrate inductors of sufficient size to meet the requirements of the circuit.
For example, a radio frequency (RF) amplifier typically employs a tuned load which has both inductive and capacitive components, to act as a secondary filter to filter out-of-band signals and noise. In addition, the tuned load can provide gain by using the LC resonance of the load to null out device parasitic capacitances at the center frequency. Previously, inductors for tuned loads and other applications have been formed on silicon substrates, usually in the form of a planar spiral. However, because the silicon substrate underlying the inductor is a semiconductor material, there is usually a significant parasitic capacitance between the inductor and the underlying substrate. For those applications requiring a relatively large inductor, this parasitic capacitance can dominate, thereby sharply reducing the self-resonant frequency.
Studies of previous techniques for fabricating monolithic inductors on silicon substrate have shown that monolithic inductors have often been limited to 10 nanohenries or less if a self-resonance beyond 2 GHz is desired. For many applications, it is highly desirable to have an inductance substantially in excess of 10 nanohenries. For example, to reduce power consumption, it is highly desirable to have an inductor of at least 100 nanohenries. Reduction of power consumption is of an extreme importance in portable, battery powered applications utilizing RF tuned circuits, such as miniature wireless communicators.
Because of this limitation on monolithic integrated inductors, applications requiring inductors on the order of 100 microhenries or more have typically used non-monolithic inductors which are separately packaged and coupled to the integrated active circuitry through package pins and printed circuit board wiring. However, the additional wiring requirements of the separate packaging have themselves caused parasitic capacitances. These parasitic capacitances are typically overcome by increasing the power consumption of the device. This is, of course, undesirable in portable, battery powered devices in which maximum battery life is sought.
It is known to fabricate a tuned amplifier having a monolithic inductor on a sapphire or GaAs substrate. Such devices are typically high frequency devices, on the order of tens to hundreds of Ghz. Because of the very high frequency of operation, the monolithic inductors need have a value of only a few nanohenries of inductance. Such values are readily achieved because GaAs material is a semi-insulating medium. As a consequence, the GaAs substrate on which the inductor is formed provides an essentially non-conducting dielectric insulator beneath the inductor. As a result, the parasitic capacitances of the inductors are very small and the self-resonances of the inductor are very high, as desired.
However, many RF applications require very sophisticated modulation techniques such as spread spectrum modulation, etc. to reduce interference. To date, GaAs fabrication techniques are not generally suited to fabrication of such complex integrated circuitry. Hence, construction of extremely complex integrated RF circuitry having a monolithic inductor on a single GaAs substrate has generally not been practical.